public member function
<future>

std::future::wait

void wait() const;
Wait for ready
Waits for the shared state to be ready.

If the shared state is not yet ready (i.e., the provider has not yet set its value or exception), the function blocks the calling thread and waits until it is ready.

Once the shared state is ready, the function unblocks and returns without reading its value nor throwing its set exception (if any).

All visible side effects are synchronized between the point the provider makes the shared state ready and the return of this function.

Parameters

none

Return value

none

Example

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// future::wait
#include <iostream>       // std::cout
#include <future>         // std::async, std::future
#include <chrono>         // std::chrono::milliseconds

// a non-optimized way of checking for prime numbers:
bool is_prime (int x) {
  for (int i=2; i<x; ++i) if (x%i==0) return false;
  return true;
}

int main ()
{
  // call function asynchronously:
  std::future<bool> fut = std::async (is_prime,194232491); 

  std::cout << "checking...\n";
  fut.wait();

  std::cout << "\n194232491 ";
  if (fut.get())      // guaranteed to be ready (and not block) after wait returns
    std::cout << "is prime.\n";
  else
    std::cout << "is not prime.\n";

  return 0;
}

Output:

checking...
194232491 is prime.


Data races

The future object is accessed.
The shared state is accessed as an atomic operation (causing no data races).

Exception safety

Calling this member function on a future object that is not valid, produces undefined behavior (although library implementations may detect this and throw future_error with a no_state error condition).

See also